This invention relates generally to methods of restoring contaminated soils and groundwater in-situ and, more particularly, to methods for removing and stabilizing soluble heavy metal contaminants in land formations to prevent the leaching and remobilization of those heavy metal contaminants by groundwater.
There is increasing concern over the hazards posed by the rising levels of heavy metals within the world's water supplies and land formations due to accidental spills, leaks, mining practices and poor disposal practices. Most heavy metals are toxic to some degree to all life-forms and can have a deleterious effect on aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death.
Accordingly, numerous methods have been proposed for the removal of heavy metal contamination from land and submarine formations. However, the application of such methods frequently is problematic, especially when applied in-situ to land and submarine formations several hundred feet underground. In addition, such methods typically fail to successfully treat both land and groundwater formations. As a result, even after treatment, significant amounts of mobile contaminants remain in the soil.
For example, it has been suggested that contaminated soil and water can be removed from its location and subsequently treated offsite. However, such methods are not only extremely costly and time consuming, they present added dangers associated with the transportation of such hazardous materials. In addition, it may not be feasible or practical from a cost standpoint to remove an entire contaminated area for treatment. Thus, significant amounts of contamination may remain even after application of such treatment methods.
Alternatively, it has been suggested that heavy-metal-containing-soils can be treated in-situ by various precipitation techniques. For example, in U.S. Pat. No. 4,354,942, issued to Kaczur et al., a method for the in-situ removal of soluble mercury from land and water areas using sulfur compounds is described. However, the use of sulfur compounds presents potential safety and health problems, and additional environmental risks.
In U.S. Pat. No. 4,418,961, issued to Strom et al., there is disclosed an in-situ method for restoring soluble heavy metal contaminants in subterranean formations that have been subjected to uranium mining. Strom et al. teach to add restoration fluid containing small amounts of CO.sub.2 to a mining site containing calcium carbonate in order to induce precipitation of calcium with the heavy metal ions, thereby stabilizing them. However, Strom's method is limited to the removal of metal anions. Moreover, this method essentially stabilizes only those contaminants which are already mobilized and does not treat solidified contaminants in the soil. Accordingly, these contaminants will eventually leach into the ground water, thus re-contaminating it after treatment.
Similarly, in U.S. Pat. No. 4,336,142, issued to Bye, there is disclosed a method for removal of heavy metals in land fills, contaminated soils and sewage sludge using gamma dicalcium silicate as a precipitant. As with Strom, this method will tend to stabilize only those contaminants which are already mobilized; therefore, substantial amounts of contamination will remain in the soil. In addition, if a substantial amount of soluble contamination is present in the land formation prior to treatment, it may be too costly and even impossible to achieve environmentally acceptable levels of heavy metals using this method.
It has further been suggested that heavy metals situated in soil can be removed using flushing techniques, wherein a site is flooded with an appropriate washing solution, the contaminants are mobilized into the flushing solution by way of solubilization, the solution is subsequently collected, removed and treated. However, flushing methods that are presently used require extremely long treatment times and large volumes of solution, thus resulting in high treatment costs. Moreover, even though significant amounts of the contaminants can be solubilized and removed, a potential exists for incomplete removal of contaminants due to heterogeneity of soil permeability. Accordingly, further contaminants from the soil can re-equilibrate with fresh ground water, resulting in continuous re-contamination of the ground water.